This invention relates to a novel graphite particle, a process for producing the same, a graphite paste using the graphite particle, a negative electrode for lithium secondary battery using the graphite paste, a process for producing the negative electrode, and a lithium secondary battery. More particularly, this invention relates to a lithium secondary battery suitable for use in portable instruments, electric cars, electric power storage, etc. and excellent in the rapid charge-discharge characteristics, the cycle characteristics, etc., and to graphite particle for use as a negative electrode thereof, a process for producing the graphite particle, a graphite paste using the graphite particle, a negative electrode for lithium secondary battery using the graphite paste, and a process for producing the negative electrode.
As prior art graphite particles, natural graphite particle, artificial graphite particle prepared by graphitization of coke, artificial graphite particle prepared by graphitization of organic polymeric material, pitch and the like, graphite particles prepared by pulverizing these graphites, etc. can be referred to. These graphite particles are put to use as a negative electrode for lithium secondary battery by mixing a graphite particle with an organic binder and an organic solvent to prepare a graphite paste, coating a copper foil surface with the graphite paste, and then evaporating the solvent. For instance, it is intended in JP-B 62-23433 to eliminate the problem of internal short-circuit caused by lithium dendrite and to improve the cycle characteristics by using graphite as negative electrode.
However, in the natural graphite particle in which graphite crystals are well grown and in the artificial graphites prepared by graphitization of coke, the interlaminar bonding force in the direction of c-axis of crystal is weaker than the bonding force in the crystal face direction, and therefore the bonding between graphite layers is broken upon pulverization to form the so-called xe2x80x9cflake graphitexe2x80x9d having a large aspect ratio. If the flake graphite particle having a great aspect ratio is kneaded together with a binder and coated onto a current collector to form an electrode, the flaky graphite particles are oriented in the plane direction of current collector. As its result, due to repeated occlusion of lithium into graphite crystal and its release, a strain arises in the direction of c-axis, which causes an internal breakage of electrode. Thus, the cycle characteristics are deteriorated and, in addition, the rapid charge-discharge characteristics tend to become worse.
Further, prior art graphite particles having a large crystallite size in the face direction requires a long period of time for occlusion and release of lithium. Further, prior flaky graphite particles having a high aspect ratio have a great specific surface area. Thus, the lithium secondary battery obtained therefrom has a large irreversible capacity in the first cycle and, in addition, such graphite particles are poor in adhesiveness to current collector so that a large quantity of binder is needed. If the adhesiveness to current collector is not good, the current-collecting effect is not good and discharge capacity, rapid charge-discharge characteristics and cycle characteristics are deteriorated. Thus, it is desired to develop a graphite particle excellent in the rapid charge-discharge characteristics and cycle characteristics, or small in the irreversible capacity in the first cycle and excellent in cycle characteristics, or small in the irreversible capacity in the first cycle and capable of improving rapid charge-discharge characteristics and cycle characteristics, in the form of a lithium secondary battery.
This invention provides graphite particles solving the problems mentioned above and suitable for use as a negative electrode of a lithium secondary battery excellent in rapid charge-discharge characteristics and cycle characteristics.
This invention further provides graphite particles suitable for use as a negative electrode of lithium secondary battery small in the irreversible capacity of the first cycle and excellent in cycle characteristics.
This invention further provides a process for producing graphite particles suitable for use as a negative electrode of a lithium secondary battery which is excellent in rapid charge-discharge characteristics and cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in the rapid charge-discharge characteristics and cycle characteristics.
This invention further provides a graphite paste suitable for use as a negative electrode of a lithium secondary battery which is excellent in rapid charge-discharge characteristics and cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in the rapid charge-discharge characteristics and cycle characteristics.
This invention further provides a negative electrode of a lithium secondary battery which has a high capacity, and is excellent in the rapid charge-discharge characteristics and cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in cycle characteristics, or small in the irreversible capacity and excellent in the rapid charge-discharge characteristics and cycle characteristics, and a process for producing said negative electrode.
This invention further provides a lithium secondary battery which has a high capacity, and is excellent in the rapid charge-discharge characteristics and cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in the cycle characteristics, or small in the irreversible capacity of the first cycle and excellent in the rapid charge-discharge characteristics and cycle characteristics.
The graphite particles of this invention have the following characteristic features (1) to (6).
(1) Graphite particles obtained by assembling or binding together a plurality of flat-shaped particles so that the planes of orientation do not become parallel to one another.
(2) Graphite particles in which aspect ratio of the graphite particle is 5 or less.
(3) Graphite particles in which specific surface area is 8 m2/g or less.
(4) Graphite particles in which the size of crystallite in the direction of c-axis (the direction of thickness) of the crystal is 500 xc3x85 or more and the size of crystallite in the direction of plane is 1,000 xc3x85 or less, both as measured by X ray broad angle diffraction.
(5) Graphite particles in which pore volume of pores having a size falling in the range of 102 xc3x85 to 106 xc3x85 is 0.4 to 2.0 cc/g based on the weight of graphite particle.
(6) Graphite particles in which pore volume of pores having a size falling in the range of 1xc3x97102 xc3x85 to 2xc3x97104 xc3x85 is 0.08 to 0.4 cc/g based on the weight of graphite particle.
This invention further relates to a process for producing graphite particles described above characterized by mixing together an aggregate (raw material) which can be graphitized, graphite, a binder which can be graphitized, and 1 to 50% by weight of a graphitizing catalyst, followed by calcination and pulverization of the mixture.
This invention further relates to a graphite paste obtained by adding an organic binder and a solvent to the above-mentioned graphite particles or graphite particles produced by the above-mentioned process, and homogenizing the mixture.
The negative electrode for the lithium secondary battery of this invention is produced by the use of the above-mentioned graphite paste, and has the following characteristic features (1) to (3).
(1) A negative electrode for a lithium secondary battery obtained by coating the above-mentioned graphite paste onto a current collector and forming an integrated body.
(2) A negative electrode for a lithium secondary battery obtained by integrating a mixture of graphite particles and organic binder and a current collector, wherein the pressed and integrated mixture of graphite particles and organic binder has a density of 1.5 to 1.9 g/cm3.
(3) A negative electrode for a lithium secondary battery obtained by integrating a mixture of graphite particles and organic binder and a current collector, wherein the content of the organic binder is 3 to 20% by weight based on said mixture.
This invention further relates to a process for producing the negative electrode for lithium secondary battery of (2), characterized by adding 1 to 50% by weight of a graphitizing catalyst to an aggregate which can be graphitized or graphite and a binder which can be graphitized, homogenizing the mixture, calcining it, pulverizing it to obtain graphite particles, adding and mixing an organic binder and a solvent to the graphite particles, coating the mixture onto a current collector, and evaporating the solvent, followed by pressing and integration.
Further, this invention relates to a lithium secondary battery comprising a casing, a cover, at least one pair of negative and positive electrodes, said casing, cover and electrodes being disposed through intermediation of separators, and an electrolytic solution provided in the neighborhood of said casing, cover and electrodes, wherein said negative electrode is produced by the use of the above-mentioned graphite particles.